In the manufacture of paper, a papermaking furnish, i.e. an aqueous slurry of cellulosic fiber having a water content of usually more than 95 wt.-% is formed into a paper sheet typically having a water content of less than 5 wt.-%. Hence, the dewatering (drainage) and retention aspects of papermaking are important to the efficiency and cost of the manufacture. Typically, a cellulosic thin stock is drained on a moving screen to form a sheet which is then dried. It is well known to apply water soluble polymers to the cellulosic suspension in order to effect flocculation of the cellulosic solids and enhance drainage on the moving screen. According to a well known method for papermaking, a cellulosic suspension is formed, flocculated by means of a flocculant, mechanically sheared, optionally re-flocculated by means of a re-flocculant, drained on a screen to form a sheet and then dried.
DE-A 44 06 624 discloses low viscous, cross-linked aqueous polymer dispersions that are prepared by polymerizing a water-soluble monomer, optionally a hydrophobic monomer and a cross-linker in the presence of a polymeric dispersant. The dispersions are useful as thickeners, flocculants and retention aids.
DE-A 195 32 229 discloses cross-linkable and cross-linked aqueous polymer dispersions that are prepared by polymerizing a water-soluble monomer, a cross-linkable N-methylol-compound, optionally a cross-linker and optionally a hydrophobic monomer in the presence of a polymeric dispersant. The polymer dispersions are useful as thickeners, flocculation aids, retention aids in papermaking, and adhesives, particularly as wallpaper coatings.
U.S. Pat. No. 5,840,804 discloses a method of producing low-viscosity water-based water-soluble polymer dispersions having a high concentration of the principal substance comprising polymerizing the following monomer components: (a1) 50-99.999 wt. % of at least one water-soluble monomer; (a2) 0.001-1 wt. % of at least one cross-linking monomer with at least two ethylenically unsaturated radically polymerizable groups; (a3) 0-30 wt. %, particularly 1-25 wt. %, of at least one hydrophobic monomer; and (a4) 0-25 wt. %, particularly 0.1-15 wt. %, of at least one amphiphilic monomer, in aqueous solution, in the presence of at least one polymer dispersant, to form a polymer (A), where the sum of the amounts of the components represented by the monomers (a1), (a2), (a3), and (a4) is 100 wt. % of the monomers, the weight average molecular weight of the resulting polymer (A) is at least 500,000 Dalton, and polymer (A) is incompatible with the dispersant.
The properties of the polymer dispersions of the prior art are, however, not satisfactory in every respect.